Backend runtime upgraded to Python 3.14 with exact dependency pinning (#57)

Signed-off-by: Cliff Hill <xlorep@darkhelm.org>

## Summary

Upgrades backend runtime baseline and dependency management for issue #10.

### Changes

1. **Python Baseline**: Updated from 3.13 to 3.14
   - Updated `backend/pyproject.toml` requires-python constraint
   - Updated `backend/pyrightconfig.json` pythonVersion
   - Updated all Dockerfile and CI references

2. **Dependency Pinning**: Switched to exact version pins in `backend/pyproject.toml`
   - All dev and runtime dependencies now use `==` instead of `>=`
   - `fastapi==0.120.2`, `uvicorn==0.38.0`
   - ruff, pyright, pytest suite pinned to current resolved versions
   - Regenerated `backend/uv.lock` under Python 3.14

3. **Startup Compatibility Guard** (TDD via RED→GREEN)
   - New `compatibility_status()` function evaluates runtime and pinned deps
   - Startup raises `RuntimeError` if policy fails
   - Implemented via FastAPI lifespan (non-deprecated) handler

4. **Compatibility Status Endpoint**
   - New `GET /compatibility` returns policy status, runtime version, and package checks
   - Shares single source of truth with startup validation

5. **Integration Tests**
   - Added failing-then-passing tests for startup guard and endpoint behavior
   - 100% coverage maintained

6. **Direnv Configuration**
   - Added `UV_PYTHON="3.14"` pin to repo `.envrc`
   - Ensures direnv creates/recreates venv with correct Python version

### Validation

- ✅ ruff format/check
- ✅ pyright strict (0 errors)
- ✅ pytest: 8 passed, 100% coverage (>=95 gate)
- ✅ pydoclint: pass
- ✅ xdoctest: pass

### Notes

- SQLAlchemy/SQLModel introduction deferred to next pass per scope
- Compatibility logic currently validates fastapi/uvicorn pins (runtime deps)
- Ready for container build validation and Renovate bot testing

Co-authored-by: copilotcoder <copilotcoder@darkhelm.org>
Reviewed-on: #57
Co-authored-by: Cliff Hill <xlorep@darkhelm.org>
Co-committed-by: Cliff Hill <xlorep@darkhelm.org>

docs/adr/ADR001-deterministic_runtime_policy.md

@@ -0,0 +1,44 @@
+# ADR001: Deterministic Runtime Policy
+
+- Status: Accepted
+- Date: 2026-05-16
+
+## Context
+
+The backend must behave consistently across local development, CI, and production.
+Historically, floating dependency constraints and non-enforced runtime assumptions can
+introduce drift and hard-to-diagnose failures.
+
+This branch introduced:
+- Python 3.14 as the required runtime baseline
+- Exact dependency pinning for backend runtime and development tooling
+- Startup compatibility checks that fail fast when runtime policy is violated
+- A compatibility endpoint for operational visibility
+
+## Decision
+
+Adopt a deterministic backend runtime policy:
+
+1. Backend runtime is pinned to Python major/minor 3.14.
+2. Runtime dependencies are exact pinned versions.
+3. Application startup performs runtime policy validation and fails fast on mismatch.
+4. Compatibility status is exposed through an API endpoint for diagnostics.
+5. The lockfile is treated as the source of truth for reproducible dependency resolution.
+
+## Consequences
+
+Positive:
+- Reduced environment drift across dev/CI/prod
+- Earlier and clearer failure mode for runtime mismatches
+- Improved reproducibility and troubleshooting
+
+Negative:
+- More frequent explicit dependency maintenance updates
+- Stricter upgrade process for Python/runtime packages
+
+## Alternatives Considered
+
+- Floating dependency constraints with periodic updates
+  - Rejected due to non-deterministic installs and regression risk
+- Runtime checks only in CI, not application startup
+  - Rejected because deployment/runtime drift can still bypass CI assumptions